1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases. More particularly, it relates to the catalyst which can efficiently purify nitrogen oxides (NO.sub.x) in the exhaust gases whose oxygen concentrations are at the stoichiometric point or more than required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) therein.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduce NO.sub.x to purify the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal catalyst ingredient selected from the group consisting of Pt, Pd and Rh and loaded on the catalyst carrier layer.
The purifying performance of the 3-way catalysts for purifying exhaust gases depends greatly on the air-fuel ratio A/F of automotive engine. For instance, when the air-fuel weight ratio is larger than 14.6, i.e., when the fuel concentration is low (or on the fuel-lean side), the oxygen concentration is high in exhaust gases. Accordingly, the oxidation reactions purifying CO and HC are active, but the reduction reactions purifying NO.sub.x are inactive. On the other hand, when the air-fuel ratio is smaller than 14.6, i.e., when the fuel concentration is higher (or on the fuel-rich side), the oxygen concentration is low in exhaust gases. Accordingly, the oxidation reactions are inactive, but the reduction reactions are active.
Moreover, when driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Consequently, the air-fuel ratio varies frequently in the range of from the values adjacent to the stoichiometric point (or the theoretical air-fuel ratio: 14.6) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving conditions such as in the above-described urban areas, it is necessary to operate the automobiles on the fuel-lean side where the air-fuel mixture containing oxygen as excessive as possible is supplied to the engines. Hence, it has been desired to develop a catalyst which is capable of adequately purifying NO.sub.x even on the fuel-lean side (i.e., in the oxygen-rich atmospheres).
In view of the aforementioned circumstances, the applicants et al. of the present invention proposed a novel catalyst in Japanese Unexamined Patent Publication (KOKAI) No. 5-168,860. On this catalyst, there are loaded an NO.sub.x storage component, such as La and the like, and Pt. In the catalyst, during the fuel-lean side driving where the exhaust gas contains oxygen excessively (i.e., in the oxygen-rich atmospheres), NO.sub.x, which includes NO in an amount of about 90% by volume and the balance of NO.sub.2 etc., especially the NO is oxidized to NO.sub.2 by Pt. Then, the NO.sub.2 is stored on the NO.sub.x storage component, such as La and the like. When the air-fuel ratio A/F of the air-fuel mixture is varied to the stoichiometric point or the fuel-rich side, the stored NO.sub.2 is released. Further, on the Pt, the released NO.sub.2 is reacted with the reducing gases, such as HC and CO, included in the exhaust gases, and thereby the NO.sub.2 is reduced and purified to N.sub.2. As a result, the NO.sub.x is inhibited from being released from the catalyst during the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving. Thus, the catalyst is improved in terms of NO.sub.x purifying performance.
When exhaust-gases-purifying catalysts are used actually, it is necessary for them to exhibit 3-way catalytic performance. That is, they are required to simultaneously carry out the oxidation of HC and CO and the reduction of NO.sub.x at the stoichiometric point. However, even when the air-fuel mixture is said to have the stoichiometric point, its air-fuel ratio A/F varies in the acceleration and deceleration during driving in urban areas. Namely, the air-fuel mixture may be put into the fuel-lean state, and thereby it may include oxygen excessively. Consequently, exhaust-gases-purifying catalysts do not cause the reduction of NO.sub.x but to carry out the adsorption of NO.sub.x. If the NO.sub.x is stored saturatedly, exhaust-gases-purifying catalysts cannot store NO.sub.x any further, but release NO.sub.x to the outside as it is.
On conventional 3-way exhaust-gases-purifying catalysts, a substance having oxygen-storage ability, such as ceria (CeO.sub.2) and the like, can be loaded together with a noble metal catalyst ingredient in order to store oxygen in the oxygen-rich atmospheres (i.e., during the fuel-lean side driving), thereby facilitating the reduction reactions of NO.sub.x. Ceria can store and release oxygen by itself. That is, ceria can store oxygen in the oxygen-rich atmospheres (i.e., during the fuel-lean side driving), and it can release oxygen in the oxygen-lean atmospheres (i.e., during the fuel-rich side driving). When ceria is loaded adjacent to a noble metal catalyst ingredient, oxygen comes in and goes out at a great rate in a large amount. Accordingly, ceria buffers or reduces the fluctuation in the air-fuel ratio A/F, and it can maximize the oxidation and reduction activity of noble metal catalyst ingredient.
Hence, one can readily think of further loading ceria together with a noble metal catalyst ingredient on a catalyst with an NO.sub.x storage component loaded. If such an attempt is made, the air-fuel ratio A/F could be fluctuated less from the stoichiometric point, and oxygen could be stored in the oxygen-rich atmospheres (i.e., during the fuel-lean side driving) to establish reducing atmospheres which contain less oxygen, thereby facilitating the reduction reactions of NO.sub.x.
However, according to a series of experiments conducted by the inventors of the present invention, it was revealed that, in catalysts on which not only an NO.sub.x storage component and a noble metal catalyst ingredient, but also ceria are loaded, the NO.sub.x conversion is adversely affected to deteriorate in reducing atmospheres when ceria is loaded adjacent to a noble metal catalyst ingredient. It is believed that the disadvantage results from the fact that ceria releases oxygen at a great rate in a large amount in reducing atmospheres, and that the thus released oxygen oxidizes HC and CO so as to decrease HC and CO which contribute to the reduction reactions of NO.sub.x.